David Allen, author of Getting Things Done and inspiration for a lot of posts 'round these parts, gives away a free four-page PDF at his website that covers his basic principals for keeping email organized. Getting specific without going too in-depth, he explains the "two minute rule," why action-able emails should be kept separate from others, and why you should create your own system—such as Gina's modified "Trusted Trio". Great reading for GTD neophytes, and a good brush-up for the rest of us.